The Excel EXPONDIST Function

The exponential distribution is a continuous probability distribution, which is often used to model the time between events.

The probability density function for the Exponential Distribution is given by the formula:

and the Cumulative Exponential Distribution is given by the formula:

where x is the independant variable and λ is the parameter of the distribution.

Basic Description

The Excel Expondist function returns the value of the exponential distribution for a give value of x. The user can specify whether the probability density function or the cumulative distribution function is used.

Note that in recent versions of Excel (Excel 2010 and later), the Expondist function has been renamed the Expon.Dist function. However, the Expondist function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.

The syntax of the Expondist function is:

EXPONDIST( x, lambda, cumulative )

Where the function arguments are:

x

-

The value at which the exponential distribution is to be evaluated (must be ≥ 0).

lambda

-

The parameter of the distribution (must be > 0).

cumulative

-

A logical argument that specifies the type of distribution to be calculated. This can be either:

TRUE

-

Calculate the cumulative distribution function

FALSE

-

Calculate the probability density function

Expondist Function Examples

Example 1 - Probability Density Function

Probability Density Exponential Distribution with λ = 0.5, 1 and 2

The above chart on the right shows the Exponential Distribution probability density function with the parameter λ set to 0.5, 1, and 2.

If you want to calculate the value of the probability density function with the parameter λ set to 1, at the value x = 0.5, this can be done, using the Excel Expondist function, as follows:

=EXPONDIST( 0.5, 1, FALSE )

This gives the result 0.60653066.

Example 2 - Cumulative Distribution Function

Cumulative Exponential Distribution with λ = 0.5, 1 and 2

The above chart on the right shows the Cumulative Exponential Distribution Functions with the parameter λ set to 0.5, 1 and 2.

If you want to calculate the value of the cumulative function with the parameter λ set to 1, at the value x = 0.5, this can be done using the Excel Expondist function, as follows: